1. Technical Field
The present invention relates to a method of manufacturing a piezoelectric vibration reed, a piezoelectric vibration reed, a piezoelectric vibrator, an oscillator, an electronic instrument, and a radio time piece.
2. Description of the Related Art
In mobile phones or portable information terminal equipment, a piezoelectric vibrator having a piezoelectric vibration reed formed of a piezoelectric material such as crystal is known as a device used for a time-of-day source, a timing source such as a control signal, or a reference signal source.
There are various types of piezoelectric vibration reeds described above are known. Known examples of such piezoelectric vibration reed include a tune-fork type piezoelectric vibration reed 100 having a pair of vibrating arm portions 110 disposed in parallel, to each other, and a base portion 120 which supports a proximal end portion of the vibrating arm portions 110 in an cantilevered manner as shown in FIG. 27. The piezoelectric vibration reed 100 is capable of vibrating the pair of vibrating arm portions 110 in the direction toward and away from each other (the direction indicated by arrows in the drawing) at a predetermined resonant frequency by applying a voltage to an excitation electrode, not shown, formed on surfaces of the pair of vibrating arm portions 110.
Incidentally, the contour of the piezoelectric vibration reed 100 is formed generally by performing an etching process on a wafer substrate formed of crystal or the like. This point will be described in brief. An etching protection film is formed on a surface of the wafer substrate, and then the etching protection film 210 on a wafer substrate 200 is patterned into contours of the piezoelectric vibration reeds 100 using Photolithography technology as shown in FIG. 28. Subsequently, using the patterned etching protection film 210 as a mask, the piezoelectric vibration reed 100 shown in FIG. 27, which includes the pair of vibrating arm portions 110 supported by the base portion 120 in a cantilevered manner, is obtained by performing the etching process on the wafer substrate 200 by Wet Etching as shown in FIG. 29.
However, when performing the etching process by Wet Etching, the speed of etching the wafer substrate 200 varies depending on the directions of the crystal axes, which is a phenomenon so-called an etching anisotropy. More specifically, in the respective crystal axes of the crystal (X-axis, Y-axis, and Z-axis), the etching speed is lowered in the order of Z-axis, +X axis, −X axis, Y-axis. Therefore, by the influence of the etching anisotropy, etching residuals are likely to be generated when the contours are formed, so that the contours of the piezoelectric vibration reeds 100 are likely to have irregular shapes. In particular, as shown in FIG. 27 and FIG. 30, etching residuals 116 are likely to be generated at a crotch portion 115 of the proximal end portions of the pair of vibrating arm portions 110 (the base portion).
This point will be described.
Normally, when forming the tuning-fork type piezoelectric vibration reed 100, the wafer substrate 200 is cut out from a crystal raw stone so that the Z-axis of the crystal axis substantially matches the thickness direction (L1 direction) of the piezoelectric vibration reed 100, the Y-axis matches the longitudinal direction (L2 direction) of the piezoelectric vibration reed 100, and the X-axis matches the width direction (L3 direction) of the piezoelectric vibration reed 100 for the purpose of obtaining desired contour by the etching process as shown in FIG. 27. However, since the etching speed varies by any means, the etching residuals 116 are likely to be generated at the crotch portion 115 between the pair of vibrating arm portions 110 as shown in FIG. 31.
In particular, as the etching process is proceeded and the point being processed approaches the portion of the crotch portion 115, a flow of etching solution is impaired by the etching protection film 210 so that the reaction of etching becomes slow. Therefore, the speed of procession of the etching process in the Y-axis direction, in which the etching speed becomes the slowest, is further lowered, whereby the etching residuals 116 are likely to be generated.
If the etching residuals 116 are generated at the crotch portion 115, the pair of vibrating arm portions 110 are resulted in unbalanced vibrations, which may result, in turn, in an increased CI value due to a change of vibration characteristics and a vibration leak.
Accordingly, by devising the shape of the etching protection film corresponding to the crotch portion while taking the difference in etching speed into account in advance, a technology to adjust the vibration balance of the pair of vibrating arm portions is known (JP-A-2005-167992).
However, even when the technology described in JP-A-2005-167992 is employed, the etching residual itself still remains significantly. Therefore, a stress tends to concentrate on coupling portions between portions of the etching residuals and the vibrating arm portions at the time of vibrations for example, and the lowering of the strength of portions where the stress is concentrated is resulted. Consequently, the corresponding portions may become a starting point of damage caused by the external impact or the like, so that the vibrating arm portions are susceptible to cracks or the like.
In addition, the etching residual may work to change the length of the pair of the vibrating arm portions, so that the change of the variation characteristic such as shifting of the resonant frequency is still likely to occur.
It is believed that the etching residuals may be reduced by performing Wet Etching for a long time. However, the time to be used for etching is increased, and hence lowering of the productivity is resulted. Furthermore, since the period to be soaked into the etching solution, which is drug solution, may increase as well, a problem that the etching protection film becomes eroded and hence a desirable etching process cannot be performed, so that the vibration characteristics may deteriorate.